Marketers can increase the level of engagement they get with Facebook fans if they do one simple thing: Join Twitter, Google+, and Pinterest as well, according to a study of advertisers using social media by Vitrue, a maker of social media management tools and apps.

The survey looked at the Facebook pages of 500 marketing clients and compared the level of fan engagement between companies that also used Twitter, Google+ and Pinterest.

It found that engagement rates double when consumers encounter brands in other social media in addition to Facebook.

Unfortunately, most marketers have a self-defeating social media strategy: Less than half of brands are on a social network other than Facebook, Vitrue found.